The present invention relates to an ultrasonic motor, particularly, to an ultrasonic motor with rotational characteristics and durability thereof being improved and a manufacturing cost thereof being reduced.
An ultrasonic motor is configured with a stator that includes a piezoelectric body with a plurality of polarized piezoelectric segments circumferentially arranged and a rotatable disc-shaped or annular rotor in contact with the stator under a predetermined pressure. In the ultrasonic motor, the piezoelectric body of the stator is vibrated with high frequency voltage being applied thereto. The induced vibration of the piezoelectric body is enhanced in a circumferential direction of the stator by a comb body provided integrally to the piezoelectric body such that the comb body is driven to induce a traveling vibration wave in the circumferential direction. Thereby, the rotor, which frictionally engages with the piezoelectric body, can be rotated around an axis thereof. The comb body has a function of enlarging amplitude of the vibration of the piezoelectric body. However, since the amplitude is generally one micrometer to three micrometers, the comb body is required to be in close contact with the rotor evenly in the circumferential and radial directions of the stator so as to improve rotational efficiency of the rotor (i.e., rotational energy of the rotor to vibration energy of the stator). Therefore, in a conventional ultrasonic motor, when machining surfaces of the comb body and the rotor that contact with each other, high machining accuracy is required so as to regulate profile irregularities and deviations from flatness thereof within several micrometers. In addition, accuracy on the same order is required with respect to a degree of parallelization between the comb body and rotor to be assembled.
In order to meet the aforementioned requirements, a technique is proposed in which there is formed on the surface of the rotor that faces the comb body a slider made of polymer resin that can easily be machined to control the profile irregularity and deviation from flatness (e.g., Japanese Patent Provisional Publication No. HEI 9-98587, hereinafter, simply referred to as '587 Publication). In '587 Publication, a technique is further proposed in which the slider is formed from conductive organic material or conductive organic material is applied onto a surface of the slider so as to prevent wear of the slider to be accelerated by static electricity generated on the slider and to prevent rotational efficiency of the rotor from being reduced due to dust adhering to the slider by the electrostatic force.
In '587 Publication, the slider, which is an additional component different from the rotor, is employed as a component of the ultrasonic motor. It leads to increase of the number of components of the ultrasonic motor and needs a further process of attaching the slider to the rotor. Thereby, a manufacturing cost of the ultrasonic motor is increased. Especially when the conductive organic material is applied onto the surface of the slider, an additional applying process is needed, and the manufacturing cost of the ultrasonic motor is further increased. Addition, in '587 Publication, since thickness of the slider or the applied material layer is not discussed, it is not clear whether rotational characteristics required for the ultrasonic motor, such as durability, the number of revolutions in a loaded state, a level of a rotation noise, and rising characteristics, are improved.